Macro slows down with each use

I have a Worksheet_SelectionChange event that runs very quickly the first time it is triggered after the file is opened. Selecting a cell brings up a calendar and upon selecting a date in the calendar, some data is processed. Each successive time the macro is triggered, it slows down even though the amount of data processed is the same. If I close the file and re-open it, the first time the macro is triggered, it runs quickly and then proceeds to slow down again with each successive run. Does anyone have any suggestions as to what could be causing this problem? Many thanks.

Cross post: https://www.mrexcel.com/forum/excel-...-each-use.html

The calendar userform is unloaded when a date is selected so I don't think that is the problem. This is the macro:

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